This invention relates generally to Sagnac effect rotation sensors and particularly to rotation sensors that sense rotation-induced phase differences between counterpropagating waves in a fiber optic sensing coil. Still more particularly, this invention relates to apparatus and methods for reducing bias errors caused by the Faraday effect in a fiber optic rotation sensor.
A fiber optic rotation sensor uses the Sagnac effect in a coil of optical fiber to detect rotations about a sensing axis that is perpendicular to the plane of the coil. Counterpropagating light waves in the sensing coil experience a phase shift that is related to the rotation rate. The phase shift is seen as a change in the interference pattern the waves make when they are combined. The interference pattern is produced when two waves of the same polarization have traversed the fiber optic sensing coil in opposite directions and then interfere. The interference pattern may be monitored by directing it onto a photodetector, which produces an electrical signal indicative of the intensity of the light in the interference fringe pattern.
It has been found that using low birefringence optical fiber, which preserves the polarization of optical signals propagating therein, and a depolarizer prevents signal fading, which is characteristic of fiber optic rotation sensors formed of low birefringence optical fiber. When a depolarizer is included in a fiber optic rotation sensor formed of high birefringence optical fiber, polarized light is split between the two orthogonal states which coincide with the principal axes of birefringence. This insures that half the light will reach the detector. A system with a perfect depolarizer will have a near zero percent degree of polarization.
Non-reciprocal phase shifts in fiber optic rotation sensors occur as a result of magnetic fields applied to the optical fiber. Magnetic fields interact with light guided by the optical fiber via the Faraday effect, which rotates the plane of polarization of the light waves. A bias uncertainty of 10 degrees has been reported to be caused by interaction between the earth's magnetic field and the light waves in a fiber optic rotation sensor. This bias uncertainty causes errors in measurements of rotations made with a fiber optic rotation sensor.
The rotation angle due to the Faraday effect is given by the product of the magnetic field intensity, the fiber length and the Verdet constant of the glass optical fiber. If a perfect, non-birefringent optical fiber is wrapped in a closed path, such as in the sensing coil of a fiber optic rotation sensor, the line integral of the magnetic field is zero according to Ampere's law because there is no current enclosed by the path. Therefore, the net rotation angle due to the Faraday effect is zero for such fibers.
The line integral of the magnetic field is zero only for a perfect fiber without internal or externally-induced birefringence. Real optical fiber typically has one or more types of birefringence. Fiber twist in a fiber optic sensing coil is one source of birefringence. Fiber twist can occur in the optical fiber during manufacturing or as a result of the coil winding process. Fiber twist in the fiber optic rotation sensor coil acts as an unavoidable phase retarder that, together with an external magnetic field, causes bias drift due to the Faraday effect. In a fiber optic rotation sensor coil, the presence of the magnetic field and the retarders results in a net bias shift between the counterpropagating waves.
Many fiber optic rotation sensor applications require an inertial measurement unit that is light in weight and low in cost. One of the many approaches for accomplishing this objective is to attempt to minimize the amount of magnetic shielding needed to reduce the fiber optic rotation sensor sensitivity to magnetic fields.
Several models have developed to describe the effects of magnetic fields on fiber optic rotation sensors. The early models provided a qualitative explanation linking twist in the fiber, birefringence of the fiber, the polarization states of the light beams propagating through the fiber and magnetic fields with the observed non-reciprocal behavior of the fiber optic Sagnac interferometer. These models provide that in order for a magnetic field to interact with a light beam traveling in a fiber, the propagation direction of the light beam and the direction of the magnetic-field must have components that are parallel. It was inferred that only transverse magnetic fields will interact with fiber optic rotation sensor coils.
However, it has been found that the fiber optic rotation sensor is sensitive to both transverse and axial magnetic fields. It is to be understood that a transverse field is in the plane of the fiber coil and that an axial field is perpendicular to the plane of the fiber coil. When the magnetic field is parallel to the coil rotation input axis, its component along the fiber axis is very small because the fiber turns have a very small projection onto the coil axis. Therefore, this field should result in a small or negligible bias drift due to the Faraday effect. Nevertheless, it has been experimentally found that many fiber optic rotation sensor coils exhibit axial magnetic field sensitivities as large or larger than their transverse sensitivities.
Compensation of the fiber gyro sensitivity to magnetic fields has been proposed previously by using a simple twisting of the fiber. This approach provides only limited compensation because it does not take into account all the variables necessary to compensate both transverse and axial fields.
Another prior art technique for reducing bias errors caused by the Faraday effect is to place the sensing coil inside a housing formed of a metal having a high magnetic permeability. The housing shields the optical fiber from magnetic fields external to the housing. Metal shields have the disadvantages of increasing both the cost and weight of the fiber optic rotation sensor. Therefore, there is a need in the art for a low cost technique for reducing bias errors caused by the Faraday effect in a fiber optic rotation sensor without adding appreciably to the weight of the rotation sensor system.